Features›Modes›Focus tracking (DECSET 1004)

Focus tracking (DECSET 1004)

Category: modes · Baseline: modern · Tags: DEC Private Modes, Xterm Extensions · Specification ↗

ESC [ ? 1004 h

DECSET 1004 enables focus in/out event reporting. The sequence is ESC [ ? 1004 h. When the terminal window gains focus, it sends ESC [ I; when it loses focus, it sends ESC [ O. Applications use this to pause animations, dim the UI, or refresh data when the user returns to the window.

How this is testedautomated
Send \x1b[?1004h (DECSET 1004), verify focusTracking mode is active.

The same probe runs against headless backends (via Termless) and real terminal apps (via a daemon launched in each terminal). This lets us distinguish parser correctness from rendering correctness.

Analysis2026-04-06

Supported by 11 of 12 terminals (92%). Not supported by: vt100.js. Part of the Modern TUI baseline. Notes: vt100.js: Not in VT100/VT220 — xterm extension.

Supported by 13 of 14 backends (93%)

Terminal Applications

Terminal	Version	Support	Notes
iTerm2	3.6.9	✓ yes	DECRPM: mode reset
Ghostty	1.3.1	✓ yes	DECRPM: mode reset
VS Code		✓ yes
Warp		✓ yes
Cursor		✓ yes
Terminal.app		✓ yes

Headless Backends

Parser correctness only — a ✓ means the parser accepts the sequence.

Backend	Version	Support	Notes
Alacritty	0.26.0	✓ yes
Kitty	0.40.0	✓ yes
vterm	0.2.0	✓ yes
WezTerm	0.1.0-fork.5	✓ yes
xterm.js	5.5.0	✓ yes
vt100.js	0.2.1	✗ no	Not in VT100/VT220 — xterm extension

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